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Check out a 2021 VERY comprehensive thesis: "<span style="color:rgb(50, 49, 48);font-family:"Segoe UI", "Segoe UI Web (West European)", "Segoe UI", -apple-system, "system-ui", Roboto, "Helvetica Neue", sans-serif;font-size:20px;font-weight:600;background-color:rgb(255, 255, 255);display:inline !important">"Investigation
into the accuracy and practicality of methods for transforming coordinates between geodetic datums (PhD thesis, book-format edition)" by Andrew Ruffhead</span> "</div>
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This has some really interesting analyses with included data sets.</div>
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Good read.</div>
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<p><font face="Copperplate Gothic Bold">Clifford J. Mugnier, c.p., c.m.s.</font></p>
<p><font face="Copperplate Gothic Bold">Chief of Geodesy, <i>Emeritus</i></font></p>
<p><font face="Copperplate Gothic Bold">LSU Center for GeoInformatics (ERAD 266)</font></p>
<p><font face="Copperplate Gothic Bold">Dept. of Civil Engineering </font></p>
<p><font face="Copperplate Gothic Bold"><b>LOUISIANA STATE UNIVERSITY</b></font></p>
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<div id="divRplyFwdMsg" dir="ltr"><font face="Calibri, sans-serif" color="#000000" style="font-size:11pt"><b>From:</b> PROJ <proj-bounces@lists.osgeo.org> on behalf of Greg Troxel <gdt@lexort.com><br>
<b>Sent:</b> Friday, November 26, 2021 11:35 AM<br>
<b>To:</b> Mathieu Poulin <mathieu.poulin@mvtgeosolutions.com><br>
<b>Cc:</b> proj@lists.osgeo.org <proj@lists.osgeo.org><br>
<b>Subject:</b> Re: [PROJ] Change of datum not working</font>
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Mathieu Poulin <mathieu.poulin@mvtgeosolutions.com> writes:<br>
<br>
> When creating a transformation that requires a change of datum, it<br>
> does not work. For instance, going from Geographic coordinates in<br>
> WGS84 to UTM coordinates in NAD83CSRS. It does convert from geographic<br>
> lat,lon to UTM projection easting,northing but the datum remains<br>
> WGS84.<br>
<br>
I think you mean "the coordinates are those I'd expect if the datum<br>
transformation did not happen".<br>
<br>
This has been discussed a lot. Briefly, WGS84 is a name for a family of<br>
datums, including WGS84(TRANSIT), the original, and the original is<br>
considered equivalent to NAD83(1986). And, no one (outside the US DoD)<br>
has high-accuracy coordinates that are truly in a WGS84 realization,<br>
because there is no high-accuracy way to access the datum. The<br>
high-accuracy techniques (carrier phase vs CORS, RTK) are all relative<br>
to either national static datums, or something in someting like ITRF2014<br>
or IGb14.<br>
<br>
There are lots of philosophical points about ensembles but my practical<br>
advice is:<br>
<br>
Surely your data is not actually in WGS84(ensemble).<br>
<br>
Beware that terminology is fuzzy with WGS84 the bare word as to<br>
whether people/programs mean:<br>
- the realization currently in use<br>
- the ensemble<br>
- the original member of the ensemble<br>
<br>
If your data was obtained from non-differential GPS observations, it<br>
would be in some specific realization of WGS84, depending on the date.<br>
If it used WAAS, it would be in the WAAS frame instead and not<br>
actually in WGS84.<br>
<br>
ITRF2008 is a good proxy for relatively recent WGS84 realizations, and<br>
I think what WAAS uses. (ITRF2014 is a good proxy for measuremnets<br>
since early 2021, in WGS84(G2139), but if you can tell the difference<br>
beteen actual measurements in ITRF2008 and ITRF2014 please tell us how<br>
:-))<br>
<br>
Keep in mind that ITRF2008 coordinates of points in Canada have<br>
non-zero velocities. So you should be thinking about epoch if you<br>
want to be cm-level. If you are trying to avoid the meter-level shift<br>
and ignoring the ~2-3 cm/year over 10 years, that seems like an<br>
intermediate reasonable approach. (FWIW, I deal with coordinates in<br>
NAD83(2011) epoch 2010.0 obtained via RTK that are accurate at the<br>
5-10 cm level, but once I move to any dynamic frame things get<br>
messier.)<br>
<br>
If your data is not accurate at the meter level, I would say it still<br>
makes sense to transform properly form NAD83 to ITRFfoo to match<br>
others and handle round-tripping, but the finer points probably don't<br>
make sense to worry about.<br>
<br>
You said NAD83(CSRS) but didn't mention which of the 7 realizations of<br>
that you are talking about. However I'm pretty sure those are<br>
intended to be static and very close the the US NAD83 realizations,<br>
and close to each other at more or less the 10 cm level.<br>
<br>
and thus my real advice is:<br>
<br>
relabel your "WGS84" data as being ITRF2008. Perhaps by a transform,<br>
which will be null, or perhaps just label it that way.<br>
<br>
Then, conversion to NAD83(CSRS) should do the datum transform.<br>
<br>
If you are still getting null transforms, transform to NAD83(CSRS)v7<br>
instead of the bare CSRS word, which might be an ensesmble, and might<br>
be the original.<br>
<br>
<br>
<br>
A related issue is that with coordinates in Massachusetts State Plane,<br>
defined in terms of NAD83, converting to ITRF2014 can end up null, but<br>
if I convert first to NAD83(2011), which is a null transform, then the<br>
transform to ITRF2014 happens.<br>
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